When you block a member, their posts are only collapsed, and they can still PM you.
I've never bothered with ignoring someone because I could still see their collapsed posts, and I'd inevitably open them up to read them. I expect others' willpower is equally inadequate, considering the hobby we're in. It would be better to not even know ignored users have posted. The Ignore feature should help prevent fights by eliminating contact with enemies, not just add an extra mouse click or layer of privacy before the vitriol spills.